Largemouth Bass Abundance and Angler Catch Rates following a Habitat Enhancement Project at Lake Kissimmee, Florida

Transcription

1 North American Journal of Fisheries Management 23: , 2003 Copyright by the American Fisheries Society 2003 Largemouth Bass Abundance and Angler Catch Rates following a Habitat Enhancement Project at Lake Kissimmee, Florida MIKE S. ALLEN* Department of Fisheries and Aquatic Sciences, The University of Florida, 7922 Northwest 71st Street, Gainesville, Florida 32653, USA KIMBERLY I. TUGEND AND MARTY J. MANN Florida Fish and Wildlife Conservation Commission, 600 North Thacker Avenue, Suite A1, Kissimmee, Florida 34741, USA Abstract. A habitat enhancement project was conducted at Lake Kissimmee, Florida, during to improve fish habitat and remove dense inshore vegetation caused by stabilized water levels. We evaluated abundance of age-1 ( 250 mm total length [TL]) and adult (fish at least 356 mm TL and all sizes of fish caught by anglers) largemouth bass Micropterus salmoides before and after the habitat enhancement. Mean electrofishing catch per hour (CPH) of age-1 largemouth bass increased significantly after the habitat enhancement, suggesting strong year-classes for 2 years after the habitat enhancement (i.e., year-classes). Growth of age-1 largemouth bass also increased following habitat enhancement; mean total length of age-1 fish averaged 143 mm before enhancement and 186 mm after enhancement. Catch curves conducted in 2001 and 2002 corroborated historical electrofishing data indicating that the 1997 and 1998 year-classes were abundant as adults compared with other year-classes in the age frequencies. Age-1 largemouth bass electrofishing catch rates were not related to seasonal water levels or coverage of hydrilla Hydrilla verticillata. Despite the rapid growth rates and high abundance of the 1997 and 1998 year-classes, neither electrofishing catch rates of largemouth bass at least 356 mm TL nor angler catch rates of largemouth bass (fish/h, all sizes of fish, harvested or released; data from creel surveys) differed significantly between preenhancement and postenhancement periods. Thus, we were unable to detect a change in adult largemouth bass abundance or angler catch rates following the habitat enhancement. Fishing effort directed toward largemouth bass declined after enhancement for the winter period (November February) but did not differ significantly between preenhancement and postenhancement periods for the summer (May August) period. Benefits of muck removal concurrent with lake drawdowns include increased recreational opportunities and improved habitat. However, our results indicate that fish population responses to drawdowns and muck removals may vary and detecting effects on the adult largemouth bass populations can be difficult. Therefore, habitat enhancement efforts should focus on lakewide recreational benefits rather than benefits to a single preferred species (e.g., largemouth bass). Habitat enhancement projects are often conducted to improve fish habitat that is lost or degraded due to anthropogenic impacts. Fish habitat degradation in lakes and reservoirs may result from reservoir aging and siltation (Jenkins and Morais 1971; Kimmel and Groeger 1986), changes in aquatic plant communities due to altered lake hydrology (Moyer et al. 1995), or changes in trophic state and associated fish community composition shifts (Cooke et al. 1993; Ney 1996). Efforts to assess fish population responses to habitat enhancement efforts are essential for determining the success of enhancement efforts and to identify realistic goals of habitat enhancements (Bradshaw 1996). * Corresponding author: msal@ufl.edu Received September 12, 2002; accepted December 17, 2002 In Florida, channelization of drainage basins for flood control purposes has reduced the magnitude of water-level fluctuations in many natural lakes, thus reducing high water levels by 20 50% in some systems (Williams et al. 1985). Stabilized water levels allowed dense emergent plants to invade the narrow zone of lake fluctuation, which led to excessive deposition of organic matter and eventual loss of littoral fish habitat (Moyer et al. 1995; Allen and Tugend 2002). These degraded vegetation communities are characterized as dense (area coverages of 100%), with extremely high plant biomass ( 50 kg/m 2 ) and poor habitat (e.g., low dissolved oxygen) for sport fishes, including largemouth bass Micropterus salmoides (Moyer et al. 1995; Allen and Tugend 2002). In an attempt to remove dense plant materials and improve fish habitat, lake drawdowns and muck (i.e., organic plant material and sediment) removals have been 845

2 846 ALLEN ET AL. conducted on some Florida lakes (e.g., Moyer et al. 1995). Evaluations of drawdown and muck removal projects in Florida Lakes have revealed improvements in habitat quality compared with control sites, but no studies have assessed lakewide fishery impacts. Moyer et al. (1995) found that sport fish electrofishing catch rates were higher in enhanced sites than in unenhanced sites at Lake Tohopekaliga, Florida following a drawdown and muck removal project. Allen and Tugend (2002) found improved habitat quality (i.e., dissolved oxygen 4 mg/l, plant biomass of 1 2 kg/m 2 ) for juvenile largemouth bass in enhanced sites compared with control sites for 3 years following a habitat enhancement at Lake Kissimmee, Florida. However, no previous studies have assessed effects of drawdowns and muck removal on lakewide abundance or angler catch rates of sport fish such as largemouth bass. Largemouth bass recruitment in lakes and reservoirs may also be related to a number of factors other than drawdowns and muck removals. High water levels may increase inshore habitat and improve largemouth bass recruitment to age-1 (Jenkins 1970; Aggus and Elliot 1975; Aggus 1979; Timmons et al. 1980; Miranda et al. 1984; Meals and Miranda 1991). Changes in abundance of aquatic macrophytes have also influenced age-0 largemouth bass abundance and year-class strength. Age-0 largemouth bass abundance and survival has been positively related to percent coverage of aquatic macrophytes (Moxley and Langford 1985; Smith and Orth 1990; Hoyer and Canfield 1996; Miranda and Pugh 1997; Tate et al. 2003). In Florida, coverage of the exotic macrophyte hydrilla Hydrilla verticillata, which often fluctuates widely in natural lakes as related to herbicide treatments, may strongly influence abundance of age-0 largemouth bass among years (Tate et al. 2003). Thus, largemouth bass year-class strength in Florida Lakes may be influenced by a number of factors, including water-level fluctuations, changes in coverage of offshore plants such as hydrilla, and changes in habitat related to the drawdown and muck removal projects. We assessed factors related to year-class strength of largemouth bass at Lake Kissimmee and evaluated electrofishing and angler catch rates of adult largemouth bass before and after a large-scale habitat enhancement project. Our objectives were to (1) use residuals from catch curves to assess relative year-class strength of fish produced before and after the habitat enhancement project, (2) use historical data to assess electrofishing catch rates of age-1 ( 250 mm TL) and harvestable-sized fish ( 356 mm TL) from periods before and after the habitat enhancement project, (3) relate residuals from the catch curve and catch per unit effort of age-1 fish to seasonal changes in water level and annual coverage of hydrilla, and (4) assess angler effort and catch rates of largemouth bass before and after the habitat enhancement. Study Site and Background A large-scale habitat enhancement project was conducted in on Lake Kissimmee, a 14,143-ha eutrophic lake (Moyer et al. 1993) located in Osceola County, Florida. The goals of this project were to improve sport fish populations, particularly largemouth bass, and to improve angler access to littoral regions of the lake. Drawdown of the lake (normal full pool is 16 m MSL) began in November 1995 and a water level of 13.7 m (MSL) was attained in March Muck was removed from about half of the 80-km shoreline via heavy equipment (bulldozers, front-end loaders, and dump trucks). Total cost of the project was US$5 6 million. The lake was refilled beginning in June Enhanced areas (a total of 431 ha of the inshore area) contained bare sand substrate after enhancement, but plant colonization of enhanced sites occurred throughout (Allen and Tugend 2002; Tugend and Allen, in press). Methods Age and growth. To identify strong age-classes in the age frequency and age at recruitment to the fishery, we assessed largemouth bass age and growth in 2001 and Largemouth bass were collected with a boat electrofishing transects in March 2001 and March and April Sample sites included portions of the entire lake shoreline and several islands. Transects were electrofished for 20 min, and after each transect all largemouth bass were measured for total length (mm). For fish less than 400 mm TL, five fish per centimeter group were returned to the laboratory where sagittal otoliths were removed and gender was determined by visually examining the gonads. Because largemouth bass greater than 400 mm TL may have highly variable ages due to growth differences between males and females (Carlander 1977), all fish exceeding 400 mm TL were returned to the laboratory for removal of otoliths and gender determination. Individual fish measuring 660 mm TL

3 LARGEMOUTH BASS CATCH AND ABUNDANCE CHANGES 847 in 2001 and 626 mm TL in 2002 were released due to their large size and value as trophy fish. Otoliths of fish ages 1 and 2 were usually read in whole view. Otoliths for older fish or those where the age was difficult to discern clearly in whole view were prepared for sectioning by mounting with super glue the whole otolith on a glass slide. Sections (about 2 3 sections, about 0.5 mm thick) were made through the focus using either a Buehler Isomet 1000 or a South Bay Tech. Inc., Model 650 low-speed saw. Sections were then mounted on glass slides using Histomount or Thermo Shandon synthetic mount. Sections from all otoliths were read with a compound microscope by three independent readers. If disagreement occurred between readers, the readers discussed the disagreement and if necessary the otolith was resectioned and the process was repeated until an age was agreed upon by all readers. Because fish were collected just before annulus formation (Crawford et al. 1989), the ages were assigned as the number of rings plus 1 for most fish. However, the outermost growth increment was evaluated on each otolith to ensure that a new annulus had not formed near the otolith radius. Fish with a ring on the otolith radius were assigned an age equal to the number of rings. Largemouth bass age frequencies in 2001 and 2002 were estimated by combining the subsampled fish less than 400 mm TL with all fish sampled greater than 400 mm. An age length key (Ricker 1975) was used to assign age based on length for fish less than 400 mm TL. The proportion of fish in the aged subsample (N 5 fish/centimetergroup) was extrapolated to the un-aged fish in each centimeter group less than 400-mm to estimate their age based on length (Ricker 1975). The resulting age frequency from the age length key was then added to the age frequency of fish larger than 400 mm. We removed year-classes from the catch curves if they had less than three fish in the age frequency (Ricker 1975). Mean total length at age was determined for each gender to assess which year-classes had recruited into the fishery (i.e., at 356 mm TL) in 2001 and We constructed catch curves (i.e., regression of log e number of fish at each age on age) based on the estimated age frequency of largemouth bass at Lake Kissimmee during 2001 and 2002 age samples. Residuals from the catch curve often reflect the relative strength of each year-class and can be related to environmental conditions influencing year-class strength (Maceina 1997). We used Studentized residuals from the catch curves to assess whether relatively strong year-classes occurred before or after the habitat enhancement project. Studentized residuals were used because they are standardized so that 95% of residual points will be between 1.96 (Zar 1984). Historical electrofishing and creel survey data. We used historical Florida Fish and Wildlife Conservation Commission (FWC) electrofishing data from transects in to assess year-class strength. Electrofishing transects were conducted at eight fixed sites throughout the lake during February, March, or April of each year. Samples were not collected in spring 1996 (i.e., sampling 1995 year-class at age 1) because the drawdown was in progress. Sites were sampled once per year from 1983 to 1992 and repeatedly (N 2 4 times/year) in 1993, 1994, and Fixed electrofishing transects were spread throughout the lake and located in vegetation adjacent to the enhanced sites. Thus, the transects were not located in degraded inshore habitats before the habitat enhancement project. Transects were 20 min in all years except 1983 and 1991, when transects were 15 min. Length frequencies were generated each year so that the length of age-1 largemouth bass could be estimated. Largemouth bass length frequencies had strong modes at mm TL in all years, and fish within the first mode of the length frequency were considered to be age 1. The upper length for age-1 fish was usually 2 3 centimetergroups greater than the first modal peak in the length frequency (DeVries and Frie 1996; Tate et al. 2003). In 2001 and 2002, ages of fish in the first mode were determined to verify that the first mode consisted of age-1 fish. Mean total length of age-1 fish collected in FWC fixed electrofishing transects was used to index growth of the largemouth bass year-classes. Repeatedmeasures analysis of variance (ANOVA) was used to test for differences in mean length of age-1 fish between preenhancement ( ) and postenhancement year-classes ( ); sites were nested within years as the subjects. The 1996 yearclass was not included in the analysis because that year-class hatched during the lake drawdown. Mean catch per hour (CPH, fish/h) for age-1 fish was determined for each transect each year. We used repeated-measures ANOVA to test for differences in mean catch per hour (CPH) of age-1 fish between preenhancement and postenhancement year-classes; sites were nested within years as the subjects in the analysis. The 1996 year-class was not included in the analysis. Values of CPH

4 848 ALLEN ET AL. were not significantly different from normal for most years (Shapiro Wilk test, P 0.05), so no log-transformations were performed before analysis. We obtained water level data from the South Florida Water Management District. Monthly mean water levels for each year were averaged for winter (January March), spring (April June), summer (July September), and fall (October December). Correlation analysis was used to assess relations between mean seasonal water level (MSL; m) and Studentized residuals from the catch curves. Mean CPH of age-1 fish for the year-classes was also correlated with seasonal water levels to assess relations. Lake Kissimmee hydrilla coverages from 1992 to 2001 were obtained from the Florida Department of Environmental Protection (FLDEP; E. Harris, personal communication). Data were collected using fathometer transects during September or October of each year. Total area (ha) of hydrilla was estimated, and percent coverage of hydrilla was found by dividing total hectares of hydrilla by the surface hectares of the lake. Correlation analysis was used to assess relations between percent hydrilla coverage, Studentized residuals from the catch curves, and mean CPE of age-1 fish from the year-classes collected in FWC fixed transects. We used two indicators to compare preenhancement and postenhancement abundance of adult largemouth bass. First, we used electrofishing CPH of harvestable fish ( 356 mm TL per 1993 regulation) from FWC electrofishing transects in We used repeated-measures ANOVA to test for differences in mean CPH of harvestable fish between preenhancement and postenhancement years, with sites nested within years as the subjects in the analysis. Second, we used nonuniform probability roving creel surveys by the FWC (K. McDaniel, FWC, unpublished data) to indicate angler catch per hour (CPH) of all largemouth bass (all sizes, both harvested and released). Mean angler CPH was plotted through time for winter (November February) and summer (May August) creel periods from 1984 to We removed years for each season when the drawdown was in progress because low water might have concentrated fish and changed angler CPH. We tested whether mean angler CPH differed before and after the habitat enhancement using a one-way ANOVA for both seasons. Fishing effort directed toward largemouth bass was also compared between preenhancement and postenhancement periods using one-way ANOVA for both seasons, after removing years when the drawdown was in progress. Results Largemouth Bass Age Frequencies We conducted 118 total timed electrofishing transects at Lake Kissimmee in March 2001 and 82 transects during March and April of Most transects ( 85%) were sampled for 20 min in both years, and transects other than 20 min (usually min) were included in the analyses. A total of 1,025 largemouth bass were collected in 2001 and 1,081 in Fish ranged from 92 to 660 mm TL in 2001 and mm in We estimated ages of 290 largemouth bass in 2001, which included 116 fish exceeding 400 mm. In 2002, we estimated age of 269 total fish with 99 exceeding 400 mm. Mean length-at-age estimates indicated that the average-sized fish from the 1997 and 1998 year-classes (i.e., after enhancement) were evident in the fishery as recruits (i.e., 356 mm TL) during 2001 and Catch-curve analyses (Figure 1) included all age-classes in 2001, but ages 8 and 10 were not used in the 2002 catch curve because of low sample size (N 3). Age-1 fish were not used in 2002 because they may not have fully recruited to the gear (i.e., lower abundance than age-2 fish; Ricker 1975). The catch curves in 2001 and 2002 indicated similar strong and weak year-classes from 1992 to In 2001, year-classes that appeared relatively strong were 1993 (before enhancement) and 1997 and 1998 (both postenhancement), as indicated by data points located above the regression line (Figure 1). Similarly, strong year-classes in the 2002 catch curve were 1993 and postenhancement years The 1995 year-class was weak in both the 2001 and 2002 catch curves. The 1994 year-class was weak in the 2001 catch curve, and in 2002 we collected only one fish from the 1994 year-class, which was not included in the catch curve but did indicate a weak year-class. Although most year-classes were consistently strong or weak in the 2001 and 2002 catch curves, there were exceptions. For example, the 1999 yearclass was slightly below the line in 2001 and above the line in 2002, and vice versa for the 1996 yearclass. Slight changes in the age frequencies caused year-classes with low residuals to change from positive to negative. Nevertheless, year-classes containing large residuals (i.e., indicating relatively strong and weak year-classes) were consistent between the years.

5 LARGEMOUTH BASS CATCH AND ABUNDANCE CHANGES 849 FIGURE 1. Catch curve of log e (number of fish) at each age plotted on ages of fish collected at Lake Kissimmee, Florida, during March 2001 (left panel) and March and April 2002 (right panel). Numbers near each point represent the year-class (e.g., year-class). The linear regression equations are shown. Ages 8 and 10 were not included in the 2002 catch curve because of low sample size ( 3 fish), and age-1 fish were not included in 2001 because they might not have fully recruited to the gear. FIGURE 2. Mean 1 SE catch per hour (CPH; top panel) and total length (TL; mm; bottom panel) of age- 1 largemouth bass collected during spring (i.e., age-1 for each year-class) at Lake Kissimmee, Florida. Yearclasses are designated by the last two digits of the year. Data were collected from eight fixed electrofishing sites by Florida Fish and Wildlife Conservation Commission personnel. Samples were not collected in spring 1996 (1995 year-class), and the 1996 year-class was omitted due to the lake drawdown. Factors Related to Year-Class Strength Mean CPH of age-1 largemouth bass from the FWC electrofishing transects increased after the habitat enhancement. Before habitat enhancement, annual mean CPH for the year-classes ranged from 3 to 26 fish/h and averaged 14 fish/h, whereas following habitat enhancement, the year-classes ranged from 8 to 58 fish/h and averaged 27 fish/h (Figure 2). Repeated-measures ANOVA showed significant differences in mean CPH between preenhancement and postenhancement periods (F 1, , P 0.001). The year-classes were strong with mean CPH of less than 30 fish/ h, but the 2000 and 2001 year-classes had lower mean CPH (both 10 fish/h). Nevertheless, the postenhancement year-classes had significantly higher mean CPH than preenhancement (Figure 2) ones, as indicated by strong year-classes in the 3 years following habitat enhancement ( ). Historical FWC data showed increased growth of age-1 largemouth bass following habitat enhancement. Annual mean length at age 1 ranged from 129 to 162 mm TL before enhancement and averaged 143 mm, whereas after enhancement, it ranged from 166 to 194 mm and averaged 186 mm (Figure 2). Repeated-measures ANOVA revealed significant differences in mean length between preenhancement and postenhancement periods (F 1, , P 0.001). Thus, growth rates of age-1 fish increased after the habitat enhancement.

6 850 ALLEN ET AL. FIGURE 3. Mean 1 SE catch per hour (CPH) of harvestable-size ( 356 mm) largemouth bass collected during spring of each year at Lake Kissimmee, Florida. Years are designated by their last two digits. Data were collected from eight fixed electrofishing sites by Florida Fish and Wildlife Conservation Commission personnel. Samples were not collected in spring 1996 (i.e., 1995 year-class). FIGURE 4. Mean catch per angler-hour (angler CPH; fish harvested and released) of all largemouth bass at Lake Kissimmee, Florida, during winter (top panel) and summer (bottom panel) sampling periods. Years for winter periods are the earlier of the 2 years involved (e.g., 1984 November 1984 to February 1985). Years are designated by their last two digits. Data are from the Florida Fish and Wildlife Conservation Commission nonuniform probability roving creel surveys. The drawdown was conducted from November 1995 (winter period) to June 1996 (summer period). Preenhancement and postenhancement periods are shown. A 356-mm minimum length limit was imposed in July Neither relative year-class strength from the catch curves nor historical CPE data from electrofishing were significantly related to seasonal water levels. Mean seasonal water levels (m; MSL) from 1992 to 2001 ranged from 14.4 to 16.0 m in winter, m in spring, m in summer, and m in fall. The 14.4-m level in winter 1996 resulted from the mean March water levels associated with the drawdown. The CPH of age-1 fish from the year-classes (Figure 3) was not related to seasonal water level (all P 0.20). Similarly, residuals from the 2001 and 2002 catch curves were not correlated with water level in any season (all P 0.20). Thus, year-class strength was not significantly related to Lake Kissimmee water levels from 1992 to Largemouth bass year-class strength at Lake Kissimmee was not significantly related to hydrilla coverage, (obtained for 1983 to 2001, except for 1985 when coverage was not measured; S. Brittain, FLDEP, personal communication). Hydrilla coverage ranged from less than 1% to 52% of the lake s surface area during The greatest hydrilla coverage (52%) occurred in 1995, but FWC electrofishing transects were not conducted in spring 1996 (i.e., age-1 of the 1995 year-class) due to the lake drawdown. Mean age-1 largemouth bass CPH was not related to hydrilla coverage for 17 lake years (P 0.75); hydrilla coverage ranged from none to 34%. Similarly, Studentized residuals from the catch curves were also not correlated to hydrilla coverage from 1992 to 2001 (P 0.75), also suggesting no significant relation between hydrilla coverage and largemouth bass year-class strength at Lake Kissimmee. Harvestable Largemouth Bass Abundance and Angler Catch and Effort Despite the increased catch rates of age-1 fish in FWC electrofishing transects for 3 years following habitat enhancement, electrofishing catch rates of harvestable largemouth bass (i.e., 356 mm TL) did not differ significantly between preenhancement and postenhancement periods. The CPH of harvestable fish ( 356 mm TL) in FWC electrofishing transects averaged about 14 fish from 1983 to 2002 (Figure 3). The CPH of harvestable largemouth bass averaged 17 fish before enhancement and 14 fish after enhancement, but the differences were not significant in the repeated-measures ANOVA (F 1, , P 0.17). The highest CPH of harvestable fish occurred in the 2 3 years before and after the habitat enhancement, but the abundance as indexed by electrofishing did not increase significantly in the 6 years following the habitat enhancement. Creel surveys by FWC personnel indicated that angler catch rates of largemouth bass did not differ before versus 4 5 years after the habitat enhancement (Figure 4). The winter creel (November

7 LARGEMOUTH BASS CATCH AND ABUNDANCE CHANGES 851 largemouth bass generally declined after the drawdown, but the decline appeared evident before the drawdown, particularly in the winter period. FIGURE 5. Mean angling effort (thousands of hours) directed toward largemouth bass at Lake Kissimmee, Florida, during winter (top panel) and summer (bottom panel) sampling periods. Years for winter periods are the earlier of the 2 years involved (e.g., 1984 November 1984 February 1985). Years are designated by their last two digits. Data are from the Florida Fish and Wildlife Conservation Commission nonuniform probability roving creel surveys. The drawdown was conducted from November 1995 (winter period) to June 1996 (summer period). Preenhancement and postenhancement periods are shown. February) mean angler CPH was 0.36 fish before the habitat enhancement and 0.30 fish after enhancement. These values did not differ in the oneway ANOVA (F 1, , P 0.14). Summer angler CPH of largemouth bass averaged 0.52 fish before and 0.63 fish after the habitat enhancement, but the differences were not significant (F 1, , P 0.20). Angling effort directed toward largemouth bass showed a decreasing trend after the early 1990s. Effort during the winter period (November February) averaged about 75,000 angler-hours before the drawdown and 32,000 angler-hours after the drawdown (Figure 5). These values differed in the one-way ANOVA (F 1, , P 0.006). However, the decrease in effort during winter appeared to begin in 1992, which preceded the drawdown. Effort during the summer period (May August) averaged 53,000 h before versus 38,000 h after the drawdown, but these values did not differ significantly (one-way ANOVA: F 1, , P 0.09). Summer effort for largemouth bass appeared similar in the years just before and after the drawdown (i.e., ), but effort during summer 2000 was the lowest mean on record (about 12,000 h). Creel surveys were not conducted in summer Thus, angler effort for Discussion Abundance and size of age-1 largemouth bass increased following the Lake Kissimmee habitat enhancement project. Electrofishing catch rates of age-1 fish in FWC fixed electrofishing transects were higher than before enhancement, and mean length at age 1 increased from about 150 mm TL to about 185 mm. Catch-curve analyses in 2001 and 2002 corroborated these results and indicated that the 1998 year-class was abundant in the population at ages 3 and 4 and the 1997 year-class at ages 4 and 5, relative to other year-classes in the population. Thus, sampling at age-1 and subsequent samples after recruitment to the fishery indicated strong year-classes in 1997 and 1998 and about average year-class strength in 1999 and Based on rapid growth rates and high abundance of the 1997 and 1998 year-classes, we expected that electrofishing catch rates of harvestable-sized fish ( 356 mm TL) would increase in , as would angler catch rates. However, neither electrofishing CPH nor angler CPH differed between preenhancement and postenhancement periods. The statistical power of the tests was probably low because of the low number of years (5 6) after habitat enhancement. Nevertheless, FWC mean annual electrofishing CPH of fish over 356 mm TL and angler catch rates appeared generally similar between pre- and postenhancement periods. Weak year-classes before the enhancement (i.e., 1994 and 1995) may have contributed to the lack of an increase in adult largemouth bass abundance after enhancement. The 1994 and 1995 yearclasses were weak, as indicated by catch-curve residuals, and low abundance of these fish may have contributed to the lack of a change in electrofishing CPH of harvestable-sized fish after enhancement. We estimated that less than 3% of the largemouth bass population in 2002 was composed of fish hatched before the drawdown (i.e., 5 years old). Conversely, in 1996, 10% of the Lake Kissimmee largemouth bass population was composed of fish older than age 5 (our unpublished data). Thus, the lack of an increase in CPH of harvestable fish may have resulted, in part, from relatively weak year-classes before the habitat enhancement. Alternatively, changes in fishing or natural mortality at Lake Kissimmee could have reduced the

8 852 ALLEN ET AL. number of young fish from the 1997 and 1998 yearclasses. Increased angler harvest (mortality) on a year-class could reduce abundance of adult fish. However, the 1997 and 1998 year-classes were abundant in both the 2001 and 2002 catch curves, indicating that these fish were still present in the population at high abundance compared with other year-classes. High natural mortality on fish produced after 1996 would not be expected because growth rates were significantly higher than before the enhancement and natural mortality of young largemouth bass is typically inversely related to fish growth (Miranda and Hubbard 1994a, 1994b; Garvey et al. 1998; Post et al. 1998). Thus, we believe it is unlikely that changes in fishing or natural mortality contributed to the lack of change in adult fish abundance estimates after the habitat enhancement. It is possible that abundance of age-1 fish was overestimated in the long-term FWC electrofishing transects after habitat enhancement due to improved catchability of larger age-1 fish after enhancement. The year-classes were significantly larger at age-1 (mean 186 mm TL) than age-1 fish produced before the enhancement (mean 143 mm). Electrofishing selects for large fish and may not effectively sample small fishes (Reynolds 1996). Bayley and Austen (2002) found that catchability (q; the proportion of fish collected with a given amount of effort) for a boat-mounted electrofisher increased greatly with largemouth bass size. They found that q determined from fixed electrofishing times in enclosed areas increased from about 0.08 to about 0.12 (i.e., 50% increase) as largemouth bass increased from 150 to 200 mm TL. Using their relations, electrofishing an area containing 200 largemouth bass would catch about 16 fish if they were 150 mm TL but 24 fish if they were 200 mm TL. The larger size of age-1 largemouth bass after habitat enhancement may have increased their vulnerability to electrofishing, which could have overestimated their abundance compared with catch rates of fish before enhancement (i.e., smaller size). Although the 1997 and 1998 year-classes still appeared to be relatively strong year-classes as adults in the catch curves, the increased size of fish after the habitat enhancement could have caused a 40 50% overestimate of their abundance at age 1, based on relations from Bayley and Austen (2002). Angler effort for largemouth bass during winter declined after habitat enhancement, based on FWC creel data, but the decline appeared evident before the habitat enhancement in Summer effort did not differ significantly between preenhancement and postenhancement periods, but the 2000 summer period produced the lowest effort on record. Some anglers, fishing guides, and marina owners claimed that the habitat enhancement project caused poor largemouth bass fishing at Lake Kissimmee (authors personal observation). We found no evidence of a decline in adult fish abundance or angler catch rates after the habitat enhancement. Angler catch rates from FWC creel surveys included some of the highest rates on record in winter 1999 (0.48 fish/h) and summer 1998 (0.88 fish/h), but these means did not differ from preenhancement levels. Thus, we did not find evidence that adult largemouth bass abundance and fishing quality declined after the habitat enhancement. Although improved largemouth bass fishing was a goal of the Lake Kissimmee habitat enhancement project, creel survey data indicated that the project did not increase angler catch rates and fishing effort directed toward largemouth bass appeared to decline. Largemouth bass year-class strength was not related to mean seasonal water-level fluctuations from 1992 to Previous studies have found a positive relation between largemouth bass yearclass strength and water levels during spring (Jenkins 1970; Aggus and Elliot 1975; Aggus 1979; Timmons et al. 1980; Miranda et al. 1984; Meals and Miranda 1991). Annual water level fluctuations at Lake Kissimmee were usually less than 1 m because of stabilized water levels, which probably did not greatly affect lake surface area or available habitat. Offshore native macrophytes at Lake Kissimmee may provide a relatively stable habitat for age-0 largemouth bass, thus reducing effects of relatively minor water-level fluctuations on recruitment. We did not detect a significant relation between age-1 largemouth bass abundance and hydrilla coverage from 1983 to 2001, and residuals from the catch curves in 2001 and 2002 were not related to hydrilla coverage from 1992 to Largemouth bass year-class strength has been positively related to percent coverage of aquatic plants (Smith and Orth 1990; Hoyer and Canfield 1996; Miranda and Pugh 1997), including hydrilla in Florida lakes (Colle and Shireman 1980; Moxley and Langford 1985; Tate et al. 2003). Thus, we expected hydrilla coverage to influence abundance of age-1 largemouth bass at Lake Kissimmee. Tate et al. (2003) found that age-0 largemouth bass electrofishing CPH was positively related to hydrilla coverage (range 0% to nearly 80%) at Lakes

9 LARGEMOUTH BASS CATCH AND ABUNDANCE CHANGES 853 Orange and Lochloosa, Florida, from 1990 to Hydrilla coverage at Lake Kissimmee ranged from 0% to 34% in our analysis. Thus, hydrilla coverage of up to 34% did not appear to influence largemouth bass year-class strength at Lake Kissimmee. Hoyer and Canfield (1996) found that abundance of age-0 largemouth bass was positively related to percent volume infested (PVI) with aquatic macrophytes in small Florida lakes ( 270 ha), but age-0 fish abundance was highly variable among lakes for a given level of PVI. Thus, age-0 largemouth bass responses to macrophyte coverage is variable in Florida Lakes (Hoyer and Canfield 1996). Management Implications Detecting fish population-level responses to large-scale habitat enhancements remains problematic for resource managers. Geiling et al. (1996) added rock substrate as spawning habitat in the Current River, Ontario, and increased spawning area used by walleyes Stizostedion vitreum nearly five times. However, they detected no increase in the catch per effort of adult walleyes after the enhancement (Geiling et al. 1996). Geiling et al. (1996) reviewed studies of walleye habitat enhancement and concluded that increases in egg deposition and early-life survival were common consequences, but they found no studies that documented a subsequent increase in adult fish abundance. Similarly, we detected improved growth and abundance of age-1 largemouth bass for at least two year-classes (1997 and 1998) at Lake Kissimmee after the habitat enhancement but did not detect an increase in harvestable largemouth bass abundance or angler catch rates. Because the habitat enhancement included a systemwide drawdown and muck removal, we do not know what largemouth bass recruitment would have been if muck removal had not been conducted. Therefore, it was not possible to separate effects of the lake drawdown from effects of muck removal because both manipulations occurred simultaneously. Lake drawdowns without muck removal have improved recruitment of sport fishes in lakes and reservoirs (reviewed by Cooke et al. 1993). Allen and Tugend (2002) found age- 0 largemouth bass densities of about fish/ ha in enhanced sites of Lake Kissimmee during summers of Assuming the total production of age-0 fish from enhanced sites was about 100 fish/ha (i.e., all age-0 fish in enhanced sites produced there, no immigration or emigration), total lakewide production in muck-removed areas (431 ha) would be about 43,000 age-0 largemouth bass per year. On a lakewide basis (14,143 ha), this translates to about three age-0 fish per hectare per year. Even if this is an order of magnitude underestimation of the number of fish produced by enhanced sites, the small proportion of lakewide area where muck was removed (3%) suggests that strong largemouth bass year-classes resulting solely from muck-removal sites is not a realistic expectation. However, benefits of muck removal concurrent with lake drawdowns include more factors than improving recruitment of a single sport fish. Muck removal opens up littoral areas for fishing and boating activity, and improves dissolved oxygen concentrations in inshore habitats and fish diversity in enhanced sites (Allen and Tugend 2002; Tugend and Allen, in press). Muck removal also improves access to the lake by homeowners and fish camps. Extended periods (e.g., years) without drawdown and muck removal in Florida lakes could potentially result in significant lakewide habitat loss and detrimental effects to fisheries. Additionally, decades of muck deposits could accumulate to the magnitude that would be too expensive or impossible to remove mechanically. We concur with Minns et al. (1996), who argued that freshwater habitat enhancement efforts should focus on ecosystem and multispecies benefits rather than benefits to a preferred species. We believe that drawdowns and muck removals will continue to be a valuable whole-lake management tool in Florida lakes. However, sport fish population responses to drawdowns and muck removals will probably vary, and the effects on adult fish abundance and angler catch rates may be difficult to detect. Acknowledgments We thank T. Bonvechio, P. Cooney, K. Dockendorf, K. Henry, G. Kaufman, B. Tate, P. Wheeler, and other FWC personnel for help with field work and data processing. K. McDaniel provided essential FWC data for Lake Kissimmee. L. Ager, P. Bettoli, D. Canfield, J. Estes, K. Henry, W. Hubbard, S. Miranda, and M. Rogers gave helpful comments on a previous draft of this manuscript. This study was funded by the Florida Fish and Wildlife Conservation Commission. This manuscript is part of the Florida Agricultural Experiment Station Journal Series Number R References Aggus, L. R Effects of weather on freshwater fish predator-prey dynamics. Pages in H.

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